1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a top nozzle incorporating improvements which limit the handling loads that can be imposed on the fuel assembly especially when it is being loaded into or removed from the reactor core.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in U.S. Pat. Nos. 3,770,583 (No. Re. 31,583) and 3,814,667 to Klumb et al and U.S. Pat. No. 4,269,661 to Kmonk et al.
Due to occasional failure of some fuel rods during normal reactor operation and in view of the high costs associated with replacing fuel assemblies containing failed fuel rods, the trend is currently toward making fuel assemblies reconstitutable in order to minimize operating and maintenance expenses. Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal and replacement of individual failed fuel rods. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod uide thimble, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in a neutron-absorbing liquid. Once removal and replacement of the failed fuel rods have been carried out on the irradiated fuel assembly submerged at a work station and after the top nozzle has been remounted on the guide thimbles of the fuel assembly, the reconstituted assembly can then be reinserted into the reactor core and used until the end of its useful life.
One type of such reconstitutable fuel assembly can be seen in the aforementioned Klumb et al patents. The fuel assembly of Klumb et al includes a top nozzle which incorporates a hold-down plate and also coil springs coaxially disposed about upwardly extending alignment posts. The alignment posts extend through an upper end or adapter plate, spaced below the hold-down plate, and are joined thereto and to the upper ends of the guide thimbles with fastener nuts located on the underside of the adapter plate. The upper hold-down plate is slidably mounted on the alignment posts and the coil springs are interposed, in compression, between the hold-down plate and the adapter plate. A radially enlarged shoulder on the upper end of each of the alignment posts reacts with a shoulder on the hold-down plate to retain the hold-down plate on the posts.
When the fuel assembly is free standing after being removed from the reactor core, the hold-down plate is held at its uppermost position along the alignment posts by the coil springs. Further upward sliding movement of the hold-down plate is prevented by contact of the plate with the enlarged shoulders on the upper ends of the alignment posts. On the other hand, when the fuel assembly is positioned in the reactor core, the hold-down plate is pressed downward by the upper core plate of the reactor core. Thus, during reactor service, the hold-down plate slidably moves downward away from its freestanding position.
Transfer of the fuel assembly between its service position in the reactor core and a location outside of the core, such as a work station for reconstitution of the fuel assembly, is accomplished by use of a conventional fuel assembly handling machine. For handling the fuel assembly, a gripper of the machine is brought into engagement with the hold-down plate and then moved in an upward direction so as to lift the fuel assembly via its top nozzle. While the gripper so supports the fuel assembly, the load passes from the gripper to the hold-down plate and therefrom to the guide thimbles via the alignment posts in view that the connection between the hold-down plate and the guide thimbles is, in effect, substantially unyielding or rigid.
The above-described type of connection of the hold-down plate with the guide thimble alignment posts in the reconstitutable fuel assembly of the Klumb et al patents imposes on the design of the fuel assembly structure the requirement that it be capable of withstanding large lifting loads, typically on the order of 6 g. These high loads are impulse type loads which are of very short duration. (The fuel assembly handling machine has a load limiting system to prevent sustained high loads on the fuel assembly.) The postulated 6 g axial load can occur when the fuel assembly is being lowered adjacent to another assembly, and it momentarily hangs up on the stationary assembly. For example, the grids interlock or the bottom nozzle of the fuel assembly being lowered catches on the top nozzle of the stationary assembly. The fuel assembly being lowered then breaks loose from its hangup and drops downwardly until it is stopped by the fuel assembly handling machine which has continued downward. The impact energy caused by this sudden drop is now absorbed by the fuel assembly structure. (The fuel handling machine is assumed to be rigid.)
Although the above-described event occurs very infrequently, the fuel assembly structure must be designed to withstand these high loads. Unfortunately, the occurrence of these high loads, however infrequent, reduces the overall reliability of the fuel assembly structure and increases the complexity of the design of the top nozzle and guide thimble connections in the fuel assembly. Consequently, a need exists for a fresh approach to fuel assembly top nozzle design with the objective of reducing the loads on the top nozzle and guide thimble joints and thereby increasing fuel assembly reliability.